I think because it's easier. The skeuomorphic designs take thinking, actual design work. The flat UI elements essentially design themselves. Of course designers are going to gravitate towards having to do less and having to think about it carefully. That's work. People hate work, and it opens you up to criticism. If you do the same as everybody else you're relatively insulated from criticism.

Flat UI is mostly crap, but it's going to take a lot of people complaining a lot more vehemently to get anything to change.